The present invention relates to a metal-air cell having a tab system that covers an air entry port of the metal-air cell prior to use.
Metal-air cells use oxygen directly from the atmosphere to produce electrochemical energy. A metal-air cell typically has a negative electrode, the anode, and contains an active material, such as zinc, and an electrolyte such as potassium hydroxide. The metal-air cell does not contain a consumable cathode material, as oxygen from the atmosphere is the active cathode material. For this reason, metal-air cells have a greater capacity for anode material relative to their size and they are used extensively in applications which require moderate drains and continuous discharge usage, such as in hearing aids. The diffusion of oxygen into the cell begins a series of chemical reactions that produce gases inside the cell and ultimately consume the anode material.
Metal-air cells typically have at least one air entry port for the ingress of oxygen into the cell and the egress of hydrogen out of the cell during operation. The air entry port is covered by a tab system prior to the use of the metal-air cell to prevent it from becoming prematurely activated. Conventional tab systems typically include at least one polymer film to control diffusion of oxygen into the air entry port of the metal-air cell and at least one adhesive to adhere the polymer film to the metal-air cell.
The tab system must have proper air permeation characteristics to provide the proper balance in the amount of oxygen that diffuses into the metal-air cell and the amount of gas that diffuses out of the cell. One problem associated with metal-air cells despite the presence of a tab system is the reduction in the shelf life and the useful discharge life of the metal-air cell. Some tab systems allow too much oxygen ingress which causes the open cell voltage (OCV) of the metal-air cell to be too great, consuming an excessive amount of active material. In other metal-air cells in which a tab system has low oxygen permeation, the OCV of the metal-air cell is too low upon removal of the tab system, which causes the user to believe that the metal-air cell is dead.